294 report — 1859. 



serve hereafter the very useful purpose of linking them together and indicating 

 their relative importance until a final report can be drawn up. 



In considering the subject which I propose to investigate, several questions 

 immediately suggest themselves, which demand attention before entering 

 upon the inquiry itself, as they relate to matters which constitute the ground- 

 work of the whole subject. Of these I shall mention a few: — 1. What is 

 solution, and in what does it differ from fusion ? 2. What special function 

 does the solvent perform, and in what do water, alcohol, ether, and carbides 

 of hydrogen differ in their solvent functions? 3. In what relation does water 

 of crystallization stand to the other constituents of a hydrated salt ? 4. When 

 hydrated salts are dissolved in water, does the saline water still remain 

 attached to the salt molecule, or does it mingle with the solvent? Or, in 

 other words, in considering the physical properties of saline solutions, are we 

 to regard them as made up of water molecules and anhydrous salt molecules, 

 or as water and complex molecules of hydrated salts ? 



These questions have not been answered. No satisfactory hypothesis has 

 even been proposed for the purpose. Neither can we hope to do so before the 

 whole subject of molecular physics shall have considerably progressed beyond 

 its present imperfect state. Still, although we cannot hope to answer those 

 questions fully, they are so important in connexion with the special object of 

 the present investigation, that they must necessarily be included as far as 

 possible with it. 



Many are inclined to consider solution as a case of chemical combination. 

 In adopting this view, we do not, however, solve the problem; but if suffi- 

 cient grounds existed for adopting it, we may consider doing so a step in 

 advance, inasmuch as it would save us from the necessity of inventing a new 

 form of force. If the test of chemical combination be, that the combining 

 bodies unite in definite proportions, solution appears at first sight not to pos- 

 sess that characteristic. It appears to me, however, that we ought to di- 

 stinguish two kinds of solution : — 1, that of liquids in liquids ; and 2, of solids 

 in liquids. Some considerations founded upon the dynamical theory of heat 

 may help us to understand this distinction. 



A ccording to that theory, the molecules of gases are so far separated as to 

 be beyond the sphere of their mutual attractions, and they are further con- 

 sidered to travel onwards in straight lines according to the ordinary laws of 

 motion. A gas may therefore freely flow into another, there being no cohe- 

 sion between the molecules, and chemical attraction only when molecules 

 which are strongly polar approach within the sphere of their attractive 

 forces. In liquids, on the other hand, the repulsive action of motion is not 

 sufficient to remove them beyond the sphere of their mutual attraction, 

 notwithstanding that each molecule has not a determinate position of equili- 

 brium, and may consequently freely change its place. 



The liquid molecules are, in fact, assumed to be in a state of vibratory, 

 rotatory, and progressive motion, so that each molecule is not permanently 

 attached to another; but the progressive motion is not sufficient to carry it 

 beyond the cohesive influence of the others. We may assume that great 

 differences exist in the character and velocity of the vibratory and rotatory 

 motions of the molecules of different fluids. The fluids, the molecules of which 

 possess the same character of motion, may consequently mingle, because the 

 molecules of each will not interfere with each other's motion. When the 

 motions of the molecules of two fluids are incompatible, they do not mingle. 

 The observations of Wilson and Swan upon the changes which one liquid 

 produces in the form of the surface of contact of another, appear to support 

 the view just stated. 



